The present disclosure relates generally to medical diagnostic techniques and equipment. It particularly concerns techniques and equipment for blood analyses. The technology particularly concerns conductance measurements, for example for making hematocrit determinations.
Hematocrit (Hct) is the volume percentage of erythrocytes in whole blood. Although the term was originally applied to the apparatus or procedure used to evaluate this percentage, it is now generally used to designate the result of the determination.
More specifically, hematocrit is defined by the ratio of the volume of packed red blood cells to the volume of whole blood. It has traditionally been determined by centrifugation. According to the centrifugation method, a sample of blood is drawn into a capillary tube which is then spun at a high rate in a centrifuge until the solid portion of the blood cells become packed together in one end of the tube. The ratio of volumes is measured by simply measuring the length of: (1) the packed blood cells; and (2) the overall length of the blood sample in the tube, and dividing length (1) by length (2). In this process, length (or volume) is cancelled out and the result is typically reported as a percentage, commonly referred to as the percent packed cell volume or % PCV. This measurement has proven useful for diagnosing and evaluating a number of conditions and diseases, for example anemias.
Hematocrit measurements have also been approximated, based upon electrical conductance measurements. The electrical conductance approach generally has involved establishing a sample flow path or cell configuration that includes an electrode arrangement typically comprising two spaced electrodes of a material inert to the blood and the conditions of the conductance measurement, typically gold. The electrodes are disposed in a precisely defined, spaced, relation, so as to enable measurement of conductance of a fluid introduced between, and in contact with them. In one typical approach, the size and position of the electrodes and the size and shape of the flow path (cell) are precisely controlled. Under such circumstances, the conductance can be measured and the hematocrit can then be calculated based upon a predetermined (empirically derived) calibration curve (for example a least squares line) relating conductance to hematocrit for the same cell.
The above described approach to evaluating hematocrit has been based upon an observation that, in general, blood cells are not very conductive. Thus, the more volume that is taken up by blood cells within the space between the electrodes, the higher is the resistance (or lower is the conductance) of the system.
In the medical industry, it has become desirable to conduct analytical evaluations using easily handled sample cartridges. Such cartridges are manufactured in lots comprising a large number of cartridges. As a result, approaches to hematocrit evaluations that do not rely upon a precise controlling, and modeling, of the size and shape of the cell have been developed. Typically for such circumstances, a conductance ratio is developed based on: (1) measuring the conductance of a standard (or calibration) material of known or predetermined conductance introduced into the volume between the two electrodes, and, (2) also measuring the conductance of a blood sample placed in the same cell or location. From this, a conductance ratio is developed and calculated; for example by dividing the sample conductance (2) by the standard solution conductance (1). The division cancels out certain factors from variations due to the specific size and shape of the cell.
The conductance ratio is then used to determine hematocrit (Hct), from a predetermined calibration curve (for example a line) for the standard calibration solution and the cell. This approach reduces the need to precisely control the size and shape of the electrodes and the flow path. Alternately stated, using a ratio or relationship between an unknown blood solution, and a known calibrant or calibration solution, and then comparing the ratio to a previously established calibration curve for the same calibrant, allows measurement of hematocrit while canceling out variability from cell size, electrode shape, etc., between manufacturing batches, etc.
Substances in blood plasma, or variations in properties of blood plasma, can influence conductivity. That is, there are blood variables that affect conductivity other than from erythrocytes. For example, the concentration of electrolytes in the blood plasma can vary greatly from sample to sample (patient to patient). This can affect the calculated hematocrit result when the above described conductance ratio approach is used, since this variable is not managed or accounted for in the approach. Alternately stated, electrolyte variability in the blood sample, for example from patient to patient, can affect the conductance measurements. However, in general it is not possible to adjust for that variability, in preparation of the calibration solution. Thus, the resulting ratio discussed in the previous paragraph will not cancel out that variable, and it will be carried over into the final hematocrit calculation.
Attempts have been made to deal with this variable. For example, according to U.S. Pat. No. 4,686,479, the concentration of electrolytes in the blood is measured; and, for the hematocrit measurement, the measured electrolyte concentration is used in performing a mathematical correction to the result from conductance measurements to determine the blood conductivity. Problems with this approach include: the inherent possible variability of the needed additional blood electrolyte sensors; and, the fact that certain electrolytes may not be measured and therefore would not be included in the correction factor.
Improvement in hematocrit measurements is desired. What is particularly desired is a convenient, reproducible, approach to provide a reasonably consistent evaluation of hematocrit, from conductance type measurements. Also, an apparatus to apply the approach, is needed.
According to the present disclosure, techniques and equipment are provided for making conductance measurements to evaluate an unknown, for example, whole blood, sample. In general, the equipment can be applied to provide: a first, unfiltered, conductance measurement cell; and, a second, filtered, conductance measurement cell. In a typical arrangement the two cells are positioned for contact by a single sample, at the same time. A typical arrangement, as described, includes using a red blood filtered cell as the second, filtered, cell and positioning the arrangement as a hematocrit measurement arrangement in fluid flow communication with a liquid sample inlet in the same cartridge.
In a typical sample analysis cartridge (including the hematocrit measurement arrangement as characterized) the second, filtered, conductance measurement cell comprises a pair of spaced electrodes positioned in, or underneath, a filter, with the filter being selected, for example, to filter red blood cells, from reaching the region between the two spaced electrodes. In such an embodiment, the second conductance measurement cell will typically comprise a pair of gold electrodes each having an electrode surface area of no more than 0.04 in2 (25.8 sq. mm.). In certain embodiments described, the electrode surface area would typically be about 0.01 in2 (6.4 sq. mm.) to 0.02 in2 (12.9 sq. mm.) inclusive. Also, in typical systems the electrodes would be spaced apart from one another by a distance of no greater than 0.005 inches (0.127 mm.); and, for certain embodiments described they would be spaced apart a distance within the range of 0.0001 inch (0.025 mm.) to 0.002 inch (0.051 mm.) inclusive. In some embodiments the electrodes of the red blood cell filtered cell are spaced apart less then 50 microns.
A variety of configurations for the first conductance measurement cell are possible. In one embodiment, the first, unfiltered, conductance measurement cell comprises a pair of electrodes positioned spaced from one another, each of which is preferably a gold electrode having an electrode surface area of no greater than 0.5 in2 (322 sq. mm.). In certain embodiments described the area would be within the range of 0.2 in2 (129 sq. mm.) to 0.3 in2 (196 sq. mm.) inclusive. For a typical arrangement the two electrodes of the first conductance cell are spaced apart from one another by distance of no greater than 1.00 inch (25.4 mm.), typically 0.25 inch (6.35 mm.) to 0.5 inch (12.7 mm.) inclusive. In an alternate embodiment, a single electrode may be positioned in the first, unfiltered, conductance measurement cell.
The measurement arrangement characterized can be conveniently positioned within a removable and replaceable sample analysis cartridge, for example a cartridge (typically having a size no greater than about 100 sq. cm., usually no greater than 80 sq. cm., and preferably 50 sq. cm., or less) that can be removably and replaceably positioned within an analytical base station, for use.
The filter for the filtered cell, may comprise a hydrogel. In an alternative, it can comprise, for example, a microporous membrane. Also, the filter can be the result of providing electrodes within the cell so close, that material such as red blood cells to be filtered cannot get between them and thus are filtered out. In this latter embodiment, the electrodes would typically be covered by a dielectric material, with a cut or slit, that operates as the filter material.
In general, according to the present disclosure, a method is provided for evaluating an unknown whole blood sample. In general, the method includes a step of measuring conductance of a known whole blood sample in a hematocrit measurement system as characterized above. A conductance value from the first, unfiltered, conductance measurement cell and a conductance value from the second, filtered, conductance measurement cell, can be correlated to determine a value, for example hematocrit, for the unknown whole blood sample. In a preferred process, the step of correlation includes correlating with conductance values determined for a known calibrant in the first, unfiltered, conductance measurement cell and the second, filtered, conductance measurement cell.
A particular technique of correlating described herein involves the following steps:
1. Measuring a conductance value (CB) in the first unfiltered cell, for an unknown whole blood sample;
2. Measuring a conductance value (CP) for the unknown whole blood sample, in the second, red blood cell filtered, conductance measurement cell;
3. Measuring a conductance value (CC) for a known conductance calibrant, in the first, unfiltered, conductance measurement cell; and
4. Measuring a conductance value (CCF) for the known conductance calibrant in the second, red blood filtered, conductance measurement cell.
5. Determining a value RB according to the formula: RB=CB/CC.
6. Determining a value RP according to the formula: RP=CP/CCF.
7. Determining a value RG according to the formula: RG=RB/RP.
8. Determining a hematocrit value based on the calculated value of RG, by comparison to an emperically derived curve.
For example, once RG has been determined, the hematocrit value can be based upon a relationship such as the following:
Hematocrit=RG(C1)+C2, 
wherein C1 and C2 are constants derived for the hematocrit measurement system using a known calibrant.
Alternate functions utilizing CP, CB, CC, CCF or even RG are possible, to calculate hematocrit. The particular approach described, is a convenient approach that is relatively straight forward to implement. The order of steps, as explained below, is not, typically, critical.